the secret service shutting down pennsylvania avenue out of an abundance of caution. last night, president obama addressed the country, stopping short of calling the blast terrorism. and asking for the prayers of the nation. >> the american people will say a prayer for boston tonight. and michelle and i said our deepest thoughts and prayers to the families and the victims in the wake of this sense tense loss. bost season a tough and resilient town. so are its people. i'm confident that bostonians will pull together, and move forward as one proud city. as they do, the american people will be with them every single step of the way. >> the bombing brought back painful memories for boston. after all, one of the planes took off from logan airport on september 11th, nearly 12 years ago. michael leiter is an nbc news national security analyst. he's the former director of the national counterterrorism center in washington. michael, thanks for joining us. where i'm sitting is about four blocks from the crime scene. but on a area about a block away is being treated as a crime s